TV Tuning in the US (was Re: [mythtv] Channel changing Bugs...)

Jay R. Ashworth jra at baylink.com
Wed Apr 16 20:56:12 EDT 2003


On Sun, Apr 13, 2003 at 07:41:00AM +0200, Henk Poley wrote:
> > Simple description of Digital TV.  Digital TV boxes take Streamed MPG2 
> > and output either to RCA/SVideo/NTSC Channel 3 in the form of a settop
> box.
> 
> Yes, I know, so? Channel 3 is used for this in the USA, so?
> 
> I don't understand why you explain the concept digital TV (via set-top
> box). I think we should treat eachother as being an alien from mars. "HBO"
> (short for HoobrokBeekaDabo?) and "Showtime" (show what? underpants? ah,
> clocks!) doesn't say a thing to me. See it like a turingtest ;-)
> 
> Someone else explained that HBO and Showtime are cable transmissions, for
> example CNN. Would CNN than be 'an HBO'? Do you use a descrambler (HBO
> box?) to see these special channels, or is "HBO and Showtime" 'just there',
> as long as you can receive cable transmissions? aka, you pay for HBO and
> Showtime, and the 'normal' ether-stations are on it for free?
> 
> I Europe ether transmissions are barely used (aka not 'normal'). Most of
> the time only for the national stations, a couple or three. Everything is
> cable broadcasted. I think because in America the cities are miles from
> eachother they can manage that two broadcast antennas do not to interfer.
> In Europe they have chosen 'the right' thing, that is, let everyone (who
> wants to use it) tune to the nearest ether frequencies.

Yeah, roughly.  

To take a different approach to the clarification, if I have a TV
monitor on which I watch US television, I might feed it a program
signal from one of many possible sources:

1)	as video from an internal broadcast (VHF/UHF) tuner.

	In this case, channel selection amounts to telling the tuner
	which frequency range to tune and demodulate.  The analog
	signals are NTSc-M, 6 Mhz.

	The channel number range is 2-13 for VHF, and 14-69 for UHF.
	It used to go to channel 83, but it was clipped (and I *think*
	I've properly remembered how far) to accommodate a wider
	bandspace for public safety comms at 700MHz.

2)	as video (or possibly as RF, usually on channel 3 or 4), from a
	"cable box".

	US cable set-top boxes (mostly, these days), do one of two
	things at any given time: they either tune an analog channel
	off the cable and demodulate it to baseband, and then
	remodulate it (again, usually to channel 3 or 4).  Or, they
	demodulate a digital stream (usually MPEG2, I gather) of
	varying bitrate, depending on the "value" of the service, and
	remodulate it to video or RF.

	The channel number range for analog cable ranges from 2-125,
	IIRC, though the mapping isn't monotonic (maps of the center
	freqs for the 3 standard types of analog cable are around on
	the net; I'd link one, but I'm in mid-defrag).  The center
	freqs are also *not* exactly the same as those of broadcast:
	they're offset in one of three patterns (IRC, HRC, and
	something else).  Devices, including analog tuner cards,
	typically have these offset charts built into the tuner
	hardware.

	The digital side is a bit different; I'll cover it below.

3)	as video (or RF) from a direct broadcast satellite (Dish or
	DirecTV) set-top box.

	This stuff is likewise MPEG2, with similar outputs.

	Now here, things get a little messier, as with digital cable.
	In the digital realm, a channel number is actually an
	*address*; it identifies which of many program streams bundled
	in a superstream the viewer wants decoded and turned into
	video.  The units I've seen use channel numbers from 1-999; to
	the best of my knowledge, there's a built in map from channel
	number to transponder number and substream number (see below at
	5).

4)	as video or RF from an over-the-air digital broadcast
	television set-top box.

	This is a cross between some of the other approaches: the tuner
	has to both select a frequency range, and then extract a
	specific stream (the subchannel) and demodulate it.

	In this case, the channel number includes both a main channel
	number (which specifies the frequency range) and a subchannel
	(which specifies which stream to select).

	
5)	in a few leftover circumstances, as video or RF remodulated
	from an analog C- or KU- band satellite receiver.
	
	In this case, the channel selection is usually {position of the
	satellite in degrees,polarization,transponder} (and in the case
	of the Ku band birds, also sometimes an MPEG substream number).

	The incoming signal here is either a 36 MHz wide FM modulated
	analog NTSC-M signal, or in some cases, MPEG digital.

5 major program sources, 5 different analog channelization systems and
several digital ones (the different digital broadcast systems use
different methods of bundling the streams into a superstream, and 3
different modulation schemes, with 3 different channel number schemas
(analog/cable/dbs - 1 number, broadcast dtv - 2 numbers, satellite -
band, position, transponder, polarization, (substream)).

And (much of) this is *just* US.

How does this apply to MythTV?  

Well, that depends on whom you are, what you have to watch, and whether
you have external tuner hardware -- and how much work you want to do.
I'm not a coder; I can capsulize what the *problems* are and the
environment in which they exist to be solved, but I'm not good enough
to cut code.

But hopefully, I can contribute something useful, at least, by
providing that analysis of the problem space.

Those are the 5 major environments of "broadcast" television as it is
provided in the US; currently available tuner cards support 1, 2, and I
*think* 3 (I've heard stuff I wasn't paying attention to).  I'm not
sure about 4, and I strongly doubt 5.

But, of course, if you're sufficiently concerned about these formats,
you can always use an external tuner to demodulate them, if you can
tune it externally... though, of course, the closest thing to a
universal control interface is IR transmit.

I hope this has clarified for some of the non USAdians "all that stuff
we just know" about TV tuning.  (Where, by we, I mean me.  :-)

If I missed (or mischaracterized) something, please ask or tell me.

Cheers,
-- jra
-- 
Jay R. Ashworth                                                jra at baylink.com
Member of the Technical Staff     Baylink                             RFC 2100
The Suncoast Freenet         The Things I Think
Tampa Bay, Florida        http://baylink.pitas.com             +1 727 647 1274

   "If you don't have a dream; how're you gonna have a dream come true?"
     -- Captain Sensible, The Damned (from South Pacific's "Happy Talk")


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